Vestigial sideband (VSB) signals that are used in certain transmissions of HDTV signal have their natural carrier wave, which would vary in amplitude depending on the percentage of modulation, replaced by a pilot carrier wave of fixed amplitude, which amplitude corresponds to a prescribed percentage of modulation. This percentage modulation can be made the same as that associated with the smallest change in symbol code level. Such VSB signals using 8-level symbol coding will be used in over-the-air broadcasting within the United States, for example, and can be used in over-the-air narrowcasting systems or in cable-casting systems. However, certain cable-casting is likely to be done using suppressed-carrier quadrature amplitude modulation (QAM) signals instead, rather than VSB signals. This presents television receiver designers with the challenge of designing receivers that are capable of receiving either type of transmission and of automatically selecting suitable receiving apparatus for the type of transmission currently being received.
A television receiver designer of ordinary skill in the art will readily observe that processing after symbol decoding is similar in receivers for the VSB HDTV signals and in receivers for the QAM HDTV signals, since the data format supplied for symbol encoding is the same in transmitters for the VSB HDTV signals and in transmitters for the QAM HDTV signals. The data recovered by symbol decoding are supplied as input signal to a data de-interleaver, and the de-interleaved data are supplied to a Reed-Solomon decoder. Error-corrected data are supplied to a data de-randomizer which regenerates packets of data for a packet decoder. Selected packets are used to reproduce the audio portions of the HDTV program, and other selected packets are used to reproduce the video portions of the HDTV program. A television receiver designer of ordinary skill in the art will readily observe also that the tuners are quite similar in receivers for the VSB HDTV signals and in receivers for the QAM HDTV signals. The differences in the receivers reside in the synchrodyning procedures used to translate the final IF signal to baseband and in the symbol decoding procedures. A television receiver designer of ordinary skill in the art will readily deduce that a receiver that is capable of receiving either VSB or QAM HDTV signals is more economical in design if it does not duplicate the similar tuner circuitry prior to synchrodyning to baseband and the similar receiver elements used after the symbol decoding circuitry. The challenge is in optimally constructing the circuitry for synchrodyning to baseband and for symbol decoding to accommodate both HDTV transmission standards and in arranging for the automatic selection of the appropriate mode of reception for the HDTV transmission currently being received.
Digital HDTV signal radio receivers are known of a type that uses double-conversion in the tuner followed by synchronous detection. A frequency synthesizer generates first local oscillations that are heterodyned with the received television signals to generate first intermediate frequencies (e.g., with 920 MHz carrier). A passive LC bandpass filter selects these first intermediate frequencies from their image frequencies for amplification by a first intermediate-frequency amplifier, and the amplified first intermediate frequencies are filtered by a first surface-acoustic-wave (SAW) filter that rejects adjacent channel responses. The first intermediate frequencies are heterodyned with second local oscillations to generate second intermediate frequencies (e.g., with 41 MHz carrier), and a second SAW filter selects these second intermediate frequencies from their images and from remnant adjacent channel responses for amplification by a second intermediate-frequency amplifier. The response of the second intermediate-frequency amplifier is supplied to a third mixer to be synchrodyned to baseband with third local oscillations of fixed frequency. The third local oscillations of fixed frequency can be supplied in 0°- and 90°-phasing, thereby implementing separate in-phase and quadrature-phase synchronous detection procedures during synchrodyning. Synchrodyning is the procedure of multiplicatively mixing a modulated signal with a wave having a fundamental frequency the same as the carrier of the modulated signal, being locked in frequency and phase thereto, and lowpass filtering the result of the multiplicative mixing to recover the modulating signal at baseband, baseband extending from zero frequency to the highest frequency in the modulating signal. Separately digitizing in-phase and quadrature-phase synchronous detection results generated in the analog regime presents problems with regard to the synchronous detection results satisfactorily tracking each other after digitizing; quantization noise introduces pronounced phase errors in the complex signal considered as a phasor. These problems can be avoided in HDTV signal radio receivers of types performing the in-phase and quadrature-phase synchronous detection procedures in the digital regime. By way of example, the response of the second intermediate-frequency amplifier is digitized at twice the Nyquist rate of the symbol coding. The successive samples are considered to be consecutively numbered in order of their occurrence; and odd samples and even samples are separated from each other to generate respective ones of the in-phase (or real) and quadrature-phase (or imaginary) synchronous detection results. Quadrature-phase (or imaginary) synchronous detection takes place after Hilbert transformation of one set of samples using appropriate finite-impulse-response (FIR) digital filtering, and in-phase (or real) synchronous detection of the other set of samples is done after delaying them for a time equal to the latency time of the Hilbert-transformation filter. The methods of locking the frequency and phase of synchronous detection and the methods of locking the frequency and phase of symbol decoding differ in the VSB and QAM HDTV receivers.
The inventors point out that these types of known digital HDTV signal radio receiver present some problem in the design of the tuner portion of the receiver because the respective carrier frequencies of VSB HDTV signals and of QAM HDTV signals are not the same as each other. The carrier frequency of a QAM HDTV signal is at mid-channel of the transmission frequencies. The carrier frequency of a VSB HDTV signal is 2.375 MHz below mid-channel frequency. Accordingly, the third local oscillations of fixed frequency, which are used for synchrodyning to baseband, must be of different frequency when synchrodyning VSB HDTV signals to baseband than when synchrodyning QAM HDTV signals to baseband. The 2.375 MHz difference in frequency is larger than that which is readily accommodated by applying automatic frequency and phase control to the third local oscillator. A third oscillator that can switchably select between two frequency-stabilizing crystals is a practical necessity. In such an arrangement, of course, alterations in the tuner circuitry are involved with arranging for the automatic selection of the appropriate mode of reception for the HDTV transmission currently being received. The radio-frequency switching that must be done reduces the reliability of the tuner. The RF switching and the additional frequency-stabilizing crystal for the third oscillator increase the cost of the tuner appreciably.
Radio receivers for receiving VSB HDTV signals, in which receiver the third mixer output signal is a final intermediate-frequency signal somewhere in the 1-8 MHz frequency range rather than at baseband, are described by the inventors in the U.S. patent applications listed below, incorporated by reference herein, and commonly assigned herewith:                Ser. No. 08/237,896 filed 4 May 1994 and entitled DIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER, AS FOR INCLUSION IN AN HDTV RECEIVER;        Ser. No. 08/243,480 filed 19 May 1994 and entitled DIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER USING RADER FILTERS, AS FOR USE IN AN HDTV RECEIVER; and        Ser. No. 08/247,753 filed 23 May 1994 and entitled DIGITAL VSB DETECTOR WITH FINAL I-F CARRIER AT SUBMULTIPLE OF SYMBOL RATE, AS FOR HDTV RECEIVER.        
The final IF signal is digitized and the synchrodyne procedures are carried out in the digital regime. Radio receivers that receive QAM signals, convert them to a final IF signal just above baseband, and synchrodyne the final IF signal in the digital regime are known; and such receivers can be adapted for receiving HDTV signals, it is believed to be evident at this time to a television receiver designer of ordinary skill in the art. In radio receivers that are to have the capability of receiving digital HDTV signals no matter whether they are transmitted using VSB or QAM, the inventors point out, conversion of the signals to final IF signals just above baseband permits the frequency of the oscillations of the third local oscillator to remain the same no matter whether VSB or QAM transmissions are being received. The differences in carrier frequency location within the channel can be accommodated in the synchrodyning procedures carried out in the digital regime.